Year 521 (DXXI) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Sabbatius and Valerius (or, less frequently, year 1274 Ab urbe condita). The denomination 521 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

Year 521 (DXXI) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Sabbatius and Valerius (or, less frequently, year 1274 Ab urbe condita). The denomination 521 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.